Such a signal processor module is already known in the art, e.g. from the U.S. Pat. No. 4,697,227. Therein, the signal processor which is referred to as tracking system, checks if the received vector is located in a square centered around the constellation point i.e. the end point of an expected vector, representing a signal which should have been received instead of the really received one. The signal processor uses the position of the received vector with respect to the expected vector to determine the attenuation and frequency offset to which the signal represented by the received vector has been submitted. More in particularly, the averaged phase angle difference between the received and expected vector is indicative of this frequency offset.
A major drawback of the shape of the square is that the phase angle difference may be very high e.g. in the case of the end point of the received vector lying in a corner of the square. An occasional high phase angle difference may result in the above mentioned averaged phase angle difference which wrongly indicates a certain frequency offset. Although the U.S. Pat. No. 4,697,227 mentions arbitrarily shaped zones, it is not described how a zone needs to be shaped in order to overcome the above mentioned drawback, nor is it described how such a zone can be achieved.